Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Cost or Amortized Cost
Due in one year or less	$ 2,105	$ 1,648
Due after one year through five years	11,450	13,603
Due after five years through ten years	10,613	8,726
Due after ten years	14,730	14,164
Total Amortized Cost Basis	38,898	38,141
Estimated Fair Value
Due in one year or less	2,153	1,665
Due after one year through five years	12,078	14,442
Due after five years through ten years	10,981	9,555
Due after ten years	16,189	16,942
Total Estimated Fair Value	$ 41,401	$ 42,604